Switched mode power supplies can be used to create a direct current (DC) voltage from an alternating current (AC) voltage by switching current through an element such as an inductor. Offline converters receive a voltage from an AC source or mains and form a bulk voltage, which may then be converted into a different voltage for use by low-voltage circuitry. Typically an AC input voltage is converted into a full-wave rectified voltage in a diode bridge rectifier and smoothed before being converted into a lower voltage. One particularly useful feature for offline converters is power factor correction. A power factor controller may be used in an offline converter to ensure that power is being efficiently delivered to a load with a high power factor by keeping the voltage and current waveforms in phase.
This typical AC-DC topology, however, requires the use of a diode bridge rectifier. The diode bridge rectifier requires four discrete high-power diodes, which are inexpensive—however they create losses and generate heat in the system, impacting the efficiency and power density. In order to overcome these problems, some recent converters have adopted bridgeless, totem-pole architectures. Bridgeless totem-pole converters use different circuit paths to deliver current to the load based on the phase of the AC input signal, and providing power factor correction in bridgeless totem pole converters while preserving their low cost has been difficult.
The use of the same reference symbols in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.